Introduction

Castor Bean, or Castor Oil Plant, Ricinus communis, is a member of the monotypic genus Ricinus of the Euphorbiaceae family. It is an aggressive invasive perennial around the world, and the subtropical and tropical Americas are no exception. Castor Bean is an extremely widespread weed throughout Mexico and is also quite common throughout the southern USA. It also grows into the temperate northeastern corner of the USA, showing that it is more than capable of one day invading the Pacific Northwest and southern Canada, even though so far, it has not been reported there. The seeds of the Castor Oil plant are extremely toxic, so much so that the ingestion of even 3 or 4 seeds can cause severe gastrointestinal distress and, in some cases, even death.

Description of Ricinus communis

Ricinus communis Leaves & Stems

Castor Oil Plant grows from 1 – 5 m tall from a large taproot with several lateral roots.

It is a somewhat woody perennial shrub in tropical and subtropical climates but can behave as a herbaceous annual in temperate climates. It has a large woody hollow stem that is often purplish and covered with white powdery wax, along with the petioles and often the leaves. The stems and branches have conspicuous nodes and ring-like scars.

Leaves are arranged alternately on the stems, and at their bases are stipules 1 – 3 cm long that unite into a sheathing bud. The round leaf stalks (petioles) are 3 – 50 cm long and support very large palmate leaves with 5 – 9 lobes that are united at the bottom, with the petiole being placed just off-center (peltate) on the backside of the leaf. Leaf blades are irregularly toothed (serrated) and are very large, from 10 – 70 cm across.

Ricinus communis Flowers & Fruits

It is a monoecious plant with separate male and female flowers on the same plant.

Flowers appear in erect terminal panicles up to 40 cm long. The male flowers are located on the base of the panicle, with the female flowers located at the top of the panicle. The male flowers lack petals and sepals and are made of clusters of many stamens in branched bundles.

The female flowers lack petals but possess sepals, but they fall off early (caducous). They have a superior soft spiny ovary with 3 styles that are red or green and are 2 cleft (divided).

Fruits are ellipsoid or sub-globose and 15 – 25 mm long. They are green when young and turn brown when mature. They are usually covered in conspicuous spines but occasionally are smooth.

The fruits hold ellipsoid seeds 9 – 17 mm long that are brittle and mottled brown with a caruncle located at the base. The seeds are extremely toxic and can be lethal, even if ingested in small quantities.

Ricinus communis Toxicity

The seeds of Ricinus communis are extremely toxic. They are one of the most potent natural biological toxins known to man. This is due to the presence of an albumin called ricin. The ingestion of just a few seeds can cause severe gastroenteritis, dehydration, liver or kidney damage, and even death.

Similar Species Frequently Confused With

There are no other species in the Ricinus genus, but there are several unrelated plants that, at first glance, are often confused with Ricinus communis. However, as none are related, they can easily be differentiated by their flowers and fruits but also as follows:

• Kalopanax septemlobus is a member of the Araliaceae family. While it is native to Asia, it has been introduced to the northeastern USA. It has similar-looking leaves but its stems are covered with very long spines, and the plant reaches heights of 30 m. Its flowers have 4 or 5 petals, and they grow in umbels.
• Tetrapanax papyrifer is native to Taiwan but has been introduced along the east and west coasts of North America and central Mexico. It grows to similar heights and has large palmate leaves, but its palmate leaves’ lobes are also divided, unlike Ricinus communis. Its flowers are produced in large umbels at the top of the plant, and the flowers have 4 or 5 petals.
• Carica papaya Papaya is native to Mesoamerica but has been widely introduced into northern Mexico and the southern USA for its edible fruit. When not in fruit, it can be differentiated by its more succulent than woody and usually unbranched stem. Its large palmate leaves are also clustered near the top of the plant, and the base is deeply cordate, not united with a peltate attachment as in Castor.
• Fatsia japonica Japanese Aralia has been introduced along the east and west coasts of North America and possibly into Mexico. It grows to similar heights with palmate leaves, but its bases are not united and do not have a peltate attachment to its petioles. They produce umbels of bisexual flowers that possess both petals and sepals.
• Cnidoscolus aconitifolius or Mala Mujer (Bad Woman) is native to Mesoamerica but has also been introduced into northern Mexico, Florida, and sporadically throughout the southern USA. It can be distinguished by its large palmate leaves that only have 3-5 lobes. It is also monecious, but its flowers are small and white.
• Jatropha gossypifolia is native from Mexico to South America but has been introduced to the southern USA. It is a smaller plant with smaller leaves 7-15 cm wide with a cordate base. It also produces flowers with purple to reddish petals.
• Manihot esculenta Cassava is native to South America but has been widely introduced throughout the sub and tropical Americas as a food plant. It usually grows to only 2 m, and its palmate leaves have only 3-5 lobes that usually have smooth edges or minutely toothed at most. Its fruit is a six-angled globose capsule.

Native Distribution of Ricinus communis

The castor plant is native to northeastern Africa but has naturalized throughout Africa, some of which may have been naturally spread, and others were aided by humans as far back as the stone age.

Habitat Types Where Ricinus communis is Found

In its native environment, Ricinus communis grows naturally in any disturbed habitat. It has a wide ecological tolerance allowing it to adapt to a multitude of conditions.

Castor Bean tolerates a wide range of soil conditions from acidic to alkaline, heavy to light, including infertile soils and shallow soils.

Its moisture requirements are moderate, and it can easily tolerate extended drought and even some inundation as long as the soil drains readily and is not submerged for extended periods of time.

Ricinus communis also tolerates a wide range of temperatures. It will survive -15 C in the winter and tolerates temperatures of 35 C in the summer.

Human Uses of Castor Bean or Castor Oil Plant

Ricinus communis has a long history of use by people. It has been cultivated for its oil for at least 6000 years, with the earliest recorded use in ancient Egypt.

The plant is used as an ornamental and botanical curiosity due to its enormous size. The oil was used both medicinally and for illumination before the use of other fuels, candles, and electricity became widespread.

Medicinally, Castor Oil is used as a purge internally as well as a cure-all in smaller doses for numerous internal ailments. Externally it is used for a variety of sores and skin conditions.

Castor Oil is also used in cosmetics, soap making, foods, as a lubricant, in paints, plastic, and linoleum. The press cake is poisonous and cannot be fed to animals, but it is often used as a fertilizer or fuel.

Distribution of Ricinus communis in North America

The species was first brought to North America in the 1700s. By 1760 it was documented as naturalized in Florida.

In Canada, Ricinus communis has not yet been recorded.

In the USA, Castor Bean has been recorded throughout many southern states, including California, Arizona, Utah, Kansas, Texas, Missouri, Louisiana, Alabama, Mississippi, Georgia, Florida, and North and South Carolina. It is also found in the northeastern states of Illinois, Michigan, Ohio, Virginia, Delaware, Maryland, New York, New Jersey, Connecticut, Massachusetts, and New Hampshire. It is also found in Hawaii.

In Mexico, Ricinus communis has been reported in every state. It is particularly abundant on the east and west coasts and in south-central Mexico. It is less common in the northern desert states but has still been reported in those locations.

Castor Bean has been introduced on every continent except Antarctica and is even widespread throughout the Pacific Islands. It is a global problem.

How Castor Bean Spreads

It is primarily spread through long distances by deliberate human introductions as an ornamental or for its industrial uses.

Short-distance dispersal occurs through the explosive release of mature seeds from the fruits.

Humans also aid in short-distance dispersal by spreading seeds in garden waste, soils, and vehicles.

Habitats at Risk of Invasion in North America

Due to its wide range of ecological tolerances, many habitats are at risk of invasion, particularly any habitat with disturbance. This means that anywhere humans develop, the land is at risk of invasion.

In addition to developed areas, it also invades grasslands, heathlands, riparian communities, and farmland. It is also commonly found on roadsides and in waste areas.

It is often a primary invader of recently burned lands.

In Spain, it has even invaded sand dunes, whereas it was previously thought unable to survive in permanently arid areas.

The only habitats not thought to currently be at risk are permanent wetlands, dense old-growth forests, and alpine habitats.

Potential Benefits of Invasion

Since Ricinus communis is extremely toxic, it provides no real wildlife value.

It also reduces biodiversity where it invades.

There are no known benefits of its invasion other than the occasional native bee that may visit its flowers.

Methods to Remove Ricinus communis

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in some local garden stores. Do not buy or transport any, and do not plant them in your yard.

If you see them being sold online or in your local garden stores, please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Physical Control of Castor Bean

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming, but it usually causes the least amount of environmental damage.

The best time to remove Ricinus communis is while the plants are still small enough to pull out by hand. Due to the plant’s toxicity, gloves should be worn even when the plant is young. Pulling young plants when the soil is moist, and the roots cling less tightly to the soil is ideal. Sometimes even medium-sized plants can be pulled by hand from moist soil.

If the soil is dry or the plants are larger, weed pullers can be useful in removing more stubborn plants. Try to get as much of the root and crown as possible to help prevent resprouting. This is especially important when the soil is dry, as the root system will often break rather than give to the pulling.

With larger plants or colonies, you will need to cut the above-ground growth down, then dig out the individual crowns or cultivate the soil repeatedly with a machine for large colonies. This will help prevent any regeneration.

Burning is not recommended as a means of control as Castor Bean is a quick regenerator on burned sites, and burning may encourage rather than discourage its growth.

Disposal of the Shrubs Once Removed

If you have plants that have seeds on them, they must either be burned or solarized. Solarizing is usually best with Ricinus communis.

However, sometimes with large patches burning is more efficient for disposing of large amounts of plant matter rather than solarizing. If burning, be aware of their ability to regenerate rapidly from seed after a fire. If the fire does not attain sufficient heat, there will be a surge in seedlings not long after the fire. The area will need ongoing monitoring to deal with seedlings as they emerge.

Otherwise, to solarize, put the shrubs under a thick black tarp or into thick black garbage bags. Then leave them in the full sun for a good 8 – 10 weeks at least to be sure that all seeds are no longer viable. Many sources recommend shorter solarization periods. However, success depends on latitude, sun exposure, daytime high temperatures, and other factors that make shorter time periods prone to failure. Leaving invasive species to solarize as long as possible is always the best. Then they can be disposed of in a landfill, but be sure to inform them of their invasive status so they can be dealt with accordingly.

Chemical Control of Castor Oil Plant

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used, species riches significantly increases.

Furthermore, there are no chemical control methods that effectively target only Ricinus communis.

Chemical control is not recommended.

Biological Control of Ricinus communis

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also usually not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii, but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach.

Because Ricinus communis is cultivated as a crop, no biological control methods are being developed for it. It is, however, prone to several pests and diseases, including mung moth, pink bollworm, seedling blight, rust spot, leaf spot, gray mold, stem canker, leaf blight, and bacterial wilt. However, these all affect other crops as well, so none are being developed to control Castor Bean.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form, this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts or seedlings. Otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Crucial

In many cases of removal, the site will need to be replanted immediately. This is because the bare soil will allow the seed bank to germinate and reinvade the patch they were removed from. Removal of single isolated individuals does not require replanting but the removal of a patch will. A replanting program should already be planned and ready to implement immediately upon the removal of the Castor Bean.

Ongoing Monitoring is Essential

In all cases of invasive Ricinus communis removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not.

If seedlings are allowed to emerge unchecked, the invasive Castor Bean will simply re-invade and take over before the native species are able to grow back in their place. Removal of young plants by physical means is always the easiest and most effective means of control.

References and Resources

CABI on Ricinus communis https://www.cabi.org/isc/datasheet/47618

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Eflora Plants of North America http://www.efloras.org/browse.aspx?flora_id=1

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Willis, Lyrae (Unpublished).  Plant Families of North America.

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If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

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Introduction

Heracleum mantegazzianum  Giant Hogweed is an invasive species in North America that is still in the early stages of expanding its range. I first encountered it when I grew up in West Sechelt, BC, Canada. It was about 30 years ago. I had no idea what it was at the time, but it just seemed out of place somehow. Not long after, I started getting interested in plants, and I looked into them and discovered that, yes, in fact, it was out of place in my environment. Fortunately for me, at the time, I had not yet touched the plant to experience the phototoxic effects. I did, many years later, accidentally touch a smaller one that was among many other plants. I quickly realized what it was and stepped back immediately, but it still left some permanent discoloration on my hand. That was just from a quick accidental touch. Many others have not been so fortunate, as it causes massive blisters, scarring, and even potential blindness. Keep your children away from it, as many have suffered from contact with it. Unfortunately, it looks very similar to a native species known as Cow Parsnip; see Similar Species Frequently Confused With below.

Description of Heracleum mantegazzianum

Heracleum mantegazzianum Leaves & Stems

Heracleum mantegazzianum is a huge monocarpic herbaceous perennial from a branched root system that goes 40-60 cm deep and 15 cm across. It reaches heights of 2-4(5) m with hollow stems 3-10 cm in diameter. The stems are covered in blister-like bumps with erect bristly hairs on them. The stems also usually have purplish splotches on them, helping to differentiate them from Cow Parsnip (see similar species below).

It has very large alternate leaves from a basal rosette. As it grows in height, the leaves get smaller up the stem. The lowermost leaves can reach up to 3 m long and 1.7 m wide when fully mature. Leaves are pinnately or ternately lobed, often deeply so, and are coarsely toothed. The above-ground growth dies back and returns after each winter until it flowers, and then it dies.

Heracleum mantegazzianum Flowers

Being monocarpic, Giant Hogweed flowers between 3-5 years of age before it sets seed and then dies. Its inflorescence is a terminal compound umbel 80(100) cm across made of 50-150 rays of bisexual flowers. Surrounding the large main umbel are up to 8 other smaller satellite umbels, which often grow taller than the main one. These satellite umbels may be all male flowers only or could be bisexual. The flowers themselves are on 10-20 mm long pedicels with white to pinkish petals up to 12 mm long each.

Heracleum mantegazzianum Fruit

Heracleum mantegazzianum fruits are flattened elliptical schizocarps 6-18 mm long and 4-10 mm wide. The fruits are narrowly winged and split into 2 mericarps, each with 3-5 elongated oil ducts.

Heracleum mantegazzianum Toxicity

Heracleum mantegazzianum is extremely phototoxic, particularly when in flower. Upon touching the plant, if then exposed to the sun, it can cause severe hives, blistering, and permanent discoloration of the skin. If the sap from the plant gets in the eyes, it can cause temporary or possibly even permanent blindness.

Similar Species Heracleum mantegazzianum is Confused With

There are a few other genera that Heracleum mantegazzianum can be confused with within North America:

• Angelica atropurpurea is native to northeast North America, but it has almost entirely purple stems and grows to 1.8 m tall. Its inflorescence is much more rounded rather than flat-topped and only up to 20 cm wide. Its leaves are also much smaller, softer, and divided into 3 parts, each with its own petiole.
• Daucus carota wild carrot is widely introduced in North America and appears similar superficially, but it also grows to much smaller sizes, has smaller flower heads, and smells like a carrot.
• Poison Hemlock Conium maculatum is sometimes confused with Giant Hogweed. However, it has tripinnate leaves that look fern-like and are very soft to the touch. It also gives off a foul odor not found in Hogweed.
• Pastinaca sativa Parsnip is occasionally confused with Giant Hogweed, but its flowers are yellow instead of white and its large leaves have long petioles, and the plant smells of parsnip.

There are also similar species in the same genus, which may be more difficult to tell apart. They can be differentiated as follows:

• Heracleum maximum (Heracleum lanatum or Heracleum sphondylium subsp. montanum, there is some debate as to its proper taxonomy) is referred to as Cow Parsnip and is native to North America. While it does look strikingly similar, it grows to only 2 m but never to the 4 m that Giant Hogweed can reach. Its leaves, while still lobed, are not as deeply cut as Giant Hogweed, and it lacks the purplish spots on its stems that are commonly found on the latter. Its flower head produces much fewer rays, only 15-50 per umbel, and the main umbel is usually no more than 30 cm across. Furthermore, Giant Hogweed is found only in some states and provinces, while Cow Parsnip is widespread throughout most of North America.
• Heracleum sphondylium German Hogweed has been sporadically introduced throughout the USA and southern Canada. It can be differentiated by its smaller size of 1.2 m tall and its pinnate leaves that are pinnately lobed rather than whole leaves that are deeply divided. The leaves are also not as sharply lobed as that of Giant Hogweed.

Native Distribution of Heracleum mantegazzianum

Heracleum mantegazzianum is native to the southern slopes of the Western Greater Caucasus of southern Russia and Georgia. It has spread much further in Russia and eastern Europe in the last century, and it is believed that at least some of this spread has been of natural origin.

Habitat Types Where Giant Hogweed is Found

In its native environment, Giant Hogweed occupies a wide range of habitats from 50-2000 m in elevation with between 1000-2000 mm of annual rainfall. It thrives in temperate continental climates with hot summers and cold winters. There it is typically found in meadows, forest edges, and clearings.

In areas where it is introduced, it also occupies a wide range of habitats and is often found growing along roadsides, train tracks, rivers, riparian areas, grasslands, meadows, and waste sites.

It grows in a wide range of soil types from heavy to light, seasonally waterlogged to well-drained, and alkaline to neutral, but less often in acidic soils. It does not tolerate wet soils that experience long periods of inundation.

Heracleum mantegazzianum is normally reported in full sun to part shade conditions in North America. However, it has also been found to grow in full shade in some areas.

Human Uses of Heracleum mantegazzianum

The seeds of Heracleum mantegazzianum are used in some Middle Eastern cooking as a spice known as golpar. In its native habitat, beekeepers use the plants for honey production. There were also some reports of people eating the cooked stems, though this is not recommended due to the phototoxicity.

Distribution of Heracleum mantegazzianum in North America

The species was first recorded in New York, USA, in 1917 and had made its way to western North America by the 1930s. It was likely brought as a garden curiosity due to its impressive size. Alternatively, it could have been brought through spice importation since its seeds are sometimes used in Middle Eastern cuisine.

In Canada, Heracleum mantegazzianum has been recorded in British Columbia, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland (excluding Labrador).

In the USA, Giant Hogweed is found in Washington and Oregon in the west and in Illinois, Michigan, Pennsylvania, North Carolina, New York, Connecticut, Massachusetts, and Maine in the eastern States.

So far, in Mexico, Heracleum mantegazzianum has not yet been reported.

Globally Giant Hogweed is also considered an invasive species in much of Europe and New Zealand, and it appears to have recently been introduced to South America.

How Giant Hogweed Spreads

Heracleum mantegazzianum is primarily spread over long-distance by deliberate human introductions as an ornamental as well the importation of the seeds as a condiment. Sometimes the seeds are also found as a contaminant in the international food trade. Its preference for riparian habitats also has enabled its spread by long distances via seeds floating down rivers.

Short-distance dispersal occurs through water, high winds, animals, and sometimes birds. Dispersal can also occur through humans transporting soil and on vehicles or clothing.

Most seeds in the field germinate the following year, but the seeds can remain viable in the soil bank for up to 7 years.

Habitats at Risk of Invasion in North America

Heracleum mantegazzianum spreads mostly through water, humans, and animals and frequently is reported along roadsides as well as riparian areas where it spreads by the movement of water. In multiple locations in North America, where it has spread via waterways along the banks, it has since moved into the adjacent woodlands, grasslands, and fields. Due to its tolerance to a wide range of elevations (50-2000 m), soil types, and sun, most habitats are at risk of invasion.

However, it will not grow in permanently wet soils or arid climates without access to water. It also generally will not penetrate deeply into dense forests, but it will grow readily in open forests and forest edges. Its range is still in the early stages of expansion in North America and will continue if left unchecked.

Impacts of Invasion

In addition to the risks of phototoxicity and potential blindness, there are other impacts of invasion. Due to its large size and phototoxic effects, it can impede access to water, trails, and recreational areas.

Giant Hogweed also negatively impacts soil dynamics because some studies have shown that it decomposes more slowly than native species resulting in a slower turnover of organic matter. The annual litter that it produces can also smother nearby native species preventing their germination or regrowth.

Studies by fisheries in the USA have shown that Heracleum mantegazzianum can shade native species in riparian areas and eventually replace them. This has been shown to increase erosion due to the herbaceous nature of the plant and the timing of flooding, leaving bare soil prone to increased erosion. This can negatively impact flooding as well as habitat for native fish.

Because Giant Hogweed grows to such massive sizes, it can shade out native vegetation anywhere it is allowed to grow. Actual studies on species richness have been conflicting, however, where it has been shown to increase in grassland habitats and a decrease in woodlands where it has invaded. Data on its impacts on species richness in North America are lacking altogether, but it is safe to assume that it will have a negative impact on most habitats.

Potential Benefits of Invasion

Heracleum mantegazzianum provides virtually no wildlife value as most wildlife does not eat the plant, and birds seldom eat the seeds. Bees do visit the flowers, however, including native bees. But as with all invasive species, if they were not present, the native bees would find other native flowers instead.

Methods to Remove Heracleum mantegazzianum

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and sometimes can still be found in your local garden stores. Do not buy or transport any Giant Hogweed, and do not plant it in your yard.

If you see them being sold online or in your local garden stores, please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Physical Control of Giant Hogweed

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming, but it usually causes the least amount of environmental damage.

Safety PPE to Use During Removal

Physical methods to remove Giant Hogweed are challenging due to the phototoxic side effects of physical contact. Proper PPE will help prevent any temporary or permanent damage to your skin or eyes. Do not attempt to handle the plant in any way without proper PPE. Gloves must always be worn while handling the plant at any stage of growth. Also, wear a thick long sleeve shirt and jeans or other suitable pants made of thick fabric to ensure that they cannot make contact with your skin. Wearing a hat with mosquito netting is also highly recommended to prevent it from coming into contact with your face. Safety glasses are also highly recommended to prevent possible temporary or permanent blindness should any sap get into your eyes. See the Affiliates link at the bottom for suitable PPE.

Dealing With Young Plants

The best time to remove Giant Hogweed is when the plants are young. At this time, they can be hand-pulled wearing gloves or using a weed puller. Try to get the root as much as possible.

Dealing With Mature Plants

If the plant is already large, this makes removal much more challenging. It becomes difficult to remove the above-ground plant growth, and the rootstock also gets quite large and deep. In this case, it is more effective to wait until the flowering year and then remove all flower heads. Because it is monocarpic, it will only flower for one season, and then the plant will die. To remove the flower heads, simply cut them off into a garbage bag while wearing gloves. Do not dispose of the flower heads without solarizing them in the garbage bag first, as the heads will continue to ripen even once cut off from the plant.

You will likely need to bring a footstool or a small ladder with you to reach the top of the larger plants. You will need to return to the site a few times that season as it often will produce new flower heads. If it has gone to seed at any time, be extra careful to cut the heads directly into the garbage bag to prevent the seeds from dropping on the ground. If there are any seeds at all, be sure to return to the site the following year to destroy any seedlings while they are still small enough to easily pull out by hand while wearing gloves.

Another method to deal with mature plants is to sever their taproot or destroy the crown by digging 10-15 cm below the soil and severing it. This method can destroy a mature plant before it flowers. This can still be challenging, however, due to the size of the plant. The above-ground growth will, in most cases, need to first be removed so that you can even get close enough to the taproot or crown to destroy it.

Mechanical Removal

Mechanical removal has been shown to be ineffective, as the plants will return from their hearty rootstock after they have been cut or mowed. Repeated mowings also have proven ineffective. Sometimes repeated mowings have been seen to turn the plant into a perennial that will flower multiple times before dying instead of the monocarpic perennial that naturally only flowers once and dies. Mechanical removal is not recommended. Burning is also ineffective for the same reason that it will likely regrow from its hearty taproot.

Disposal of the Shrubs Once Removed

If you have plants that have seeds on them, they must either be burned or solarized. If you are unable to burn in your area, then you must solarize them. To solarize, put the shrubs under a thick black tarp or into thick black garbage bags and leave them in the full sun for a good 8-10 weeks at least to be sure that all seeds are no longer viable. Some sources recommend shorter solarization periods but differential heat in the bags or under the tarps, cloud cover, and other temperature variances make shorter periods less reliable. That is why it is important they are solarized for as long as possible if using that method.

Chemical Control of Giant Hogweed

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used, species riches always significantly increase.

Furthermore, there are no chemical control methods that effectively target only Giant Hogweed. And because it often grows in or near riparian areas, chemical control cannot be used there due to risks to fish and humans.

Chemical control is not recommended.

Biological Control of Heracleum mantegazzianum

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. These control methods are often extremely risky and should only be carried out by professionals after years of rigorous study. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also usually not native to the environment and could behave differently when released there.

The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective.

At this time, the only known biological control method of Heracleum mantegazzianum is the use of intensive grazing by sheep or goats and rooting by pigs. Cattle will selectively avoid eating it, as with most invasive species, so they are not a suitable control method.

Using sheep, goats, and pigs can be challenging, however, because of the phototoxicity issue. Animals with thick, dark pelts are more resistant to toxic side effects. The area of control will need to be penned in, and the sheep or goats be allowed to graze for the first season to destroy the above-ground growth. The following year pigs can be let in to root out the roots and eat them as well as any sprouts as they come up. If pigs are unavailable, then the sheep or goats can be left in there for multiple seasons, and an integrated approach can be used for stubborn plants that keep re-sprouting. In this case, you will need to use physical control by manually destroying the crown or severing the taproot using a shovel.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form, this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts or seedlings otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Crucial

In many cases of removal, the site will need to be replanted because the bare soil will allow the seed bank of this and other potentially invasive species to germinate. Single isolated plants can simply be destroyed or dealt with using physical control, and then native species will regrow in their absence. Occasionally, however, Giant Hogweed will produce a large patch. In those cases, a replanting program should be planned and ready to implement upon removal.

Ongoing Monitoring is Essential

In all cases of invasive Giant Hogweed removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not. Seeds will also continue to germinate for several years if the plant was fully established in the area and not an isolated case. Seedling removal is by far the easiest method of control. The area should be monitored once a year for several years after the plants were removed to ensure they do not become re-established.

References and Resources

CABI on Heracleum mantegazzianum https://www.cabi.org/isc/datasheet/26911

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Fire Effects Information System on Giant Hogweed https://www.fs.fed.us/database/feis/plants/forb/herman/all.html

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Willis, Lyrae (Unpublished).  Plant Families of North America.

Currently Seeking Funding To Continue This Non-Profit, Ad-Free Work

If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

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Introduction

Ilex aquifolium goes by several common names, including English Holly, European Holly, Common Holly, and Christmas Holly. It is a highly invasive plant in the moist climates of North America, particularly on the west coast, where it is beginning to dominate the understory of some forest ecosystems. Despite this, it is still widely sold in stores throughout this region.

I grew up on the coast of BC, Canada, and remember finding it all the time under the canopy of the forest. I recently returned to the coast and spent some time there and was horrified by the amount it appears to be spreading and replacing our native understory species. In a single forest walk, I managed to pull out about 40 immature plants from the forest canopy. There were several larger ones I could not pull out by hand due to their deep and aggressive root system.

Next time I return to the coast and go for a forest walk, I will be sure to bring a trowel or a small shovel. Do your part on your forest walks and pull out any small holly shrubs you find growing before they get large and too difficult to remove. Just make sure you are not pulling Oregon Grape or native hollies if you live on the east coast; see Similar Species below for more information.

Description of Ilex aquifolium

Leaves & Stems of Ilex aquifolium

Common Holly is a slow-growing evergreen perennial shrub or small tree that grows from 5-25 m tall. However, it usually only grows to the shorter 5-10 m shrub height. It may grow from a single trunk, or it may be multi-stemmed. Their trunks are usually less than 40 cm in diameter. Larger trunks can be from 40-80 cm in diameter, but they rarely exceed 100 cm. The bark is green when young but turns grey when it matures. While specimens as old as 500 years have been reported, it seldom lives longer than 100 years.

The leaves of English Holly are arranged alternately on the branches. They are tough, thick, and glossy green and measure 3-12 cm long and 2-6 cm wide. The margins are often characteristically undulate (wavy) and toothed with spiny tipped teeth. Sometimes the leaf margins are smooth, particularly on older specimens.

Flowers & Fruits of Ilex aquifolium

English Holly is a dioecious plant meaning that there are separate male and female plants. The flowers are whitish and have 4 petals and 4 sepals that are both fused near their base. In males, the flowers are usually more yellowish and appear in axillary groups (in clusters in the leaf axils). Female specimens have single flowers or in groups of three, and the flowers or white or somewhat pink. The male and female shrubs are impossible to tell apart until they begin to flower from 4 – 12 years of age.

Fruits are small red or yellow drupes, berry-like in appearance, and only grow on female plants. They are 6-10 mm in diameter. Each drupe contains 3-4 small stony seeds.

Toxicity of Ilex aquifolium

The drupes are toxic to humans and pets, causing vomiting and diarrhea, but they are generally not lethal. Birds typically eat them later in the winter after the frosts have softened them and made them less bitter; they are immune to their toxic effects.

Similar Species Ilex aquifolium is Frequently Confused With

There are a few other plant genera that have similar leaves to Ilex aquifolium. The Oregon-Grape genus Berberis of which we have a few native species growing in North America have evergreen spiny-toothed leaves, but their leaves are opposite rather than alternate, and they have blue-colored berries rather than red or yellow drupes. Osmanthus heterophyllus or Holly Olive is a member of the olive family native to Europe that also has similar leaves and has been introduced in the USA. Holly Olive differs from true hollies by its opposite rather than alternate leaves and bark that turns grayish to blackish with age and has a characteristic cracking pattern to it.

The other Ilex species can be distinguished as follows:

• Ilex opaca American Holly – native to the eastern and south-central USA. It has matte yellowish-green leaves that are seldom glossy, as in Ilex aquifolium. Its trunk is grey with wart-like bumps, and its mature branches turn brown rather than grey. It also grows more of a tree than a shrub, with trunk diameters commonly of 50 cm and up to 120 cm in diameter.
• Ilex cornuta Chinese Holly – native to China but widely introduced throughout the eastern and south-central USA. It has more rectangular-shaped leaves with only 5 (4) spines, but they are glossy green like Ilex aquifolium. Chinese Holly never grows to tree size, it rarely exceeds 3 m in height, and its drupes are larger than European Holly. The bark is smooth and grey but becomes flaky with age.
• Ilex vomitoria Yaupon Holly – native to southeastern North America. It is a small tree or shrub with glossy green leaves, but the leaves are much smaller and are not spiny. Instead, the leaves have crenate or coarsely serrated margins.

Native Distribution of Ilex aquifolium

European or Common Holly is native to western and southern Europe, northwest Africa, and southwest Asia.

Habitat Types Where European Holly is Found

European or Common Holly is typically found growing under forest canopies, oak, and beech, especially in its native range. It typically requires moist and shady environments, making it particularly suitable as an undergrowth species in any moist forest. Also, it is common on moist and shady slopes, cliffs, and mountain gorges. Sometimes, however, it can also form dense thickets in more open fields and meadows. It is a rugged pioneer species that can tolerate both frosts and droughts.

Human Uses of Common Holly

European or Common Holly is widely used as an ornamental shrub or small tree in gardens. It is also extremely popular in Christmas decorations, where it is put in wreaths, garlands, and various other displays.

In traditional European herbal medicine, the leaves of Ilex aquifolium were used as a diuretic, to treat fevers, and as a laxative. It is seldom used in modern herbal medicine, however, and its berries are considered toxic, so the plant should be used with caution.

Distribution of Ilex aquifolium in North America

In Canada, Ilex aquifolium has been recorded as introduced in BC and possibly in Ontario. It is becoming particularly invasive in coastal BC.

In the USA, English Holly is found in Washington, Oregon, and California in the west and Virginia and New Jersey in the east. It has also been introduced in Hawaii. According to iNaturalist, it is located in many more states, but many may still be in cultivation, and some could be native Ilex spp that have been misidentified.

In Mexico, Ilex aquifolium has not yet been reported so far.

English Holly has been introduced on every continent. It seems particularly bad in New Zealand, Tasmania, and North America.

How English Holly Spreads

It is primarily spread long-distance by deliberate human introductions as an ornamental species in gardens.

Short-distance dispersal occurs through birds that eat the abundant drupes later in winter after they have softened and their taste improves. They spread the seeds through their feces, where the seeds then germinate in any suitable habitat in both shade and sun.

English Holly also spreads readily vegetatively through suckering. This is how most of the thickets of Common Holly form and how it becomes a dominant understory species of the forests.

Habitats at Risk of Invasion in North America

The way that English Holly spreads through the feces of birds puts all habitats at risk providing there is enough moisture. They will not grow in permanently wet wetlands, and they also will not grow in arid or semi-arid areas. Any moderately moist to the wet climate, however, is at risk of invasion.

Their ability to grow in both sun and shade makes them a common invader of forests in particular but also roadsides, fields, meadows, cliffs, gorges, and anywhere else as long as there is ample moisture.

European Holly has proven to be particularly invasive in the temperate rainforests of northwestern North America, where it easily invades the mixed forest canopies and thrives in the moist environment there.

Impacts of Invasion

Ilex aquifolium has deep and aggressive roots that tend to out-compete the native species for nutrients and water. There is also evidence that English Holly alters the soil conditions by adding sulfur and large amounts of organic matter to the soil, which makes the conditions less hospitable for native species.

English Holly forms dense thickets that can dominate the understory of the forest and out-shade native shrubs and other native plants, reducing the biodiversity of the forests it invades.

Potential Benefits of Invasion

Thickets of Ilex aquifolium are known to be used as a refuge by small birds and deer. The flowers are attractive to bees, small butterflies, wasps, and flies, and the berries provide an important late-winter food source. However, as with all invasive plants, the native wildlife would have depended on the native plants that grew there before the Common Holly replaced them.

Methods to Remove English Holly

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in most local garden stores. Do not buy or transport any European Holly, and never plant it in your yard.

If you see them being sold online or in your local garden stores, please inform them of their invasive status and ask them to do their part and stop selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

If you live in the eastern part of North America, why not grow one of the two native hollies instead, Ilex opaca or Ilex vomitoria? If you live in western North America and want some spiny glossy green foliage, try growing native Berberis species instead. They have lovely foliage and beautiful blue-colored berries. The native Ilex and Berberis species can even be substituted for Common Holly in Christmas decorations.

Physical Control of European Holly

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming but causes the least amount of environmental damage.

Physical methods to remove European Holly generally involve uprooting the plants. This is best done when the plants are young before their aggressive roots have had a chance to establish themselves. Fortunately, they are a slow-growing species, so on a single walk through the forest, you can single-handedly destroy dozens of young plants with very little effort. Larger plants will need to be dug out with a shovel to get their roots. If the roots are not dug out, they can re-grow from the stump.

For larger shrubby plants, use pruners to remove the upper growth before using a shovel to dig out the root mass. For tree forms of Ilex aquifolium you can try girdling it. This involves removing a strip of bark from all the way around the circumference of the tree. As with all trees and woody shrubs, the only part of the tree’s trunk that is alive and transporting nutrients is the layer directly under the bark. If you cut down to the heartwood and remove the bark in a strip all around the tree, it can no longer transport water and nutrients to the leaves. The tree will then die and can be left to rot or be removed.

The best time to remove European Holly is before it is in berry, in the spring or early summer. If you need to remove them when in berry, be sure to clip off the berries before removing the shrub or tree and place them in the bag for proper disposal.

Disposal of the Shrubs Once Removed

Trees or shrubs without berries on them can be burned, cut into firewood, or left to rot. If you have plants that have mature berries on them, they must either be burned or solarized. If you are unable to burn in your area, then solarizing is a viable option. To solarize, put the shrubs under a thick black tarp or into thick black garbage bags. Then leave them in the full sun for a good 8 weeks at least to be sure that all seeds are no longer viable. The plants can then be disposed of. They can be brought to a garbage dump where you can inform them they are an invasive species so they can deal with them accordingly.

Chemical Control of European Holly

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used, species riches significantly increase.

Furthermore, there are no chemical control methods that effectively target only European Holly. If you do use chemical control, it must be stem injected. The tough evergreen leaves readily resist the absorption of pesticides making foliar applications ineffective.

Chemical control is generally not recommended.

Biological Control of Ilex aquifolium

Biological control involves the use of a predator, herbivore, disease, or some other biological agent to control an invasive species once it is fully established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective for some invasive species.

Unfortunately, there are no current methods of biological control for English Holly in North America other than goats. Domestic goats are aggressive and non-selective browsers. They will literally eat anything. This will work best on shrubs and thickets. Often goats will eat the bark off of trees so that they can even kill off the trees if they manage to eat all the way around the tree.

To use goats, the patch should be penned in, and then the goats left there to graze for at least 2 years. They will eventually destroy all the above-ground growth and then eat the new sprouts as they come up. This can then be followed with rootstock removal for any stubborn plants that may remain. Alternatively, simply leave the goats in the area for another year until you see no more sprouts. At this time, they can be moved to another patch, and that patch can be replanted with native vegetation. An ongoing monitoring program should then be implemented to be sure they do not sprout again.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest form, this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts or seedlings. Otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Sometimes Crucial

Removal of single plants from the understory of the forest will not require replanting with native species. Simply remove and monitor, and other plants will grow in their place. However, in cases of the removal of thickets that dominate the area, the site should be replanted with native species. A replanting program should already be planned and ready to implement once the English Holly has been removed. You will need to research species that are native to your area. Ideally, use ones from your local environment to ensure local ecotypes are represented. Alternatively, you can purchase suitable plants from a nursery in your area that specializes in native species.

Ongoing Monitoring is Essential

In all cases of invasive European Holly removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not. Sometimes roots left behind will re-sprout into new plants. Furthermore, birds will keep bringing in new seedlings if there are still plants in the wider surrounding environment. For these reasons, yearly monitoring should be put in place to remove young plants before they have a chance to become established, especially since that is the easiest time to remove them. With ongoing monitoring, we can keep invasive species in check and promote local biodiversity in the process.

References and Resources

Best Management Practices for English Holly in the Metro Vancouver Region http://www.metrovancouver.org/services/regional-planning/PlanningPublications/EnglishHollyBMP.pdf

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia on Ilex aquifolium https://en.wikipedia.org/wiki/Ilex_aquifolium

Willis, Lyrae (Unpublished).  Plant Families of North America.

Currently Seeking Funding To Continue This Non-Profit, Ad-Free Work

If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

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Introduction

Himalayan Blackberry or Rubus armeniacus or Rubus bifrons (confusingly, both names seem to be currently accepted) is a well-known invasive species in some areas. Where I grew up on the Sunshine Coast of British Columbia, Canada, it was everywhere. It does less well in the colder interior, but it still does grow there, just much less invasively. As a child, I did not understand that it was invasive and not native. I just loved eating handfuls of the huge juicy, and sweet berries. I would go out picking bucketfuls of berries every summer with my mother and my three brothers. Now I know how invasive it truly is, and somehow the fruits seem a little less sweet to me now. Once established, the removal of Himalayan Blackberry can be very challenging.

Description of Rubus bifrons or Rubus armeniacus

Leaves & Stems of Rubus bifrons or Rubus armeniacus

Rubus bifrons and Rubus armeniacus are members of the Rosaceae family of the Rosales Order of flowering dicots. They are a perennial vine-like trailing shrub that grows 1 – 7 m in length and 0.4 – 1 cm in diameter with arching or creeping stems that are sparse to densely hairy and armed with numerous prickles. They have a large root crown of roots and rhizomes that grow to 0.5 m and sometimes up to 2 m deep.

The leaves are deciduous or sometimes somewhat evergreen and palmately compound with 3 – 5 spreading leaflets. Leaflets are elliptic or ovate to sub-orbiculate and 6 -15 cm long and 4 – 9 cm wide with a rounded or somewhat cordate base. Margins are moderately to coarsely serrated, and the apex is acute to acuminate. The lower surface is whitish to grayish green, while the upper surface is glossy bright green or dark green. There are filiform or linear stipules that are 7 – 15 mm in length. The largest veins have hooked prickles on them.

Flowers & Fruits of Rubus bifrons or Rubus armeniacus

Himalayan Blackberries usually have loose terminal inflorescences that generally extend past their subtending leaves. Sometimes the inflorescences may grow in the leaf axils as well. They are in a thryse with 10-60(-100) flowers in them. Pedicels are covered with prickles, are densely hairy, and may or may not be glandular. Flowers are bisexual with 5 white or pink petals. Petals are not connate (free), are obovate to elliptical and shape, and are 10 – 15 mm long each. It has filiform stamen filaments, and its ovaries are apically hairy.

They produce abundant black fruits that are juicy drupecetums, an aggregation of drupes on a fleshy accessory peduncle. Most people refer to them as ‘berries’. Fruits are globose or almost cylindrical and 1 – 2 cm long with 15 – 50 individual drupelets that strongly adhere to the accessory peduncle. Abundant fertile seeds are produced in their fruits, from 7000 – 13000 seeds per square meter of stems. Seeds can remain viable in the soil for several years.

Similar Species Frequently Confused With

There are native Rubus species throughout North America, but many are smaller plants with smaller or differently shaped and/or colored berries. Some native and introduced Rubus, however, grow to similar sizes but can be differentiated as follows:

• Rubus laciniatus, Cutleaf or Evergreen blackberry, is native to Eurasia and has been fairly widely introduced in North America. It grows in similar habitats to similar sizes with similar fruits but can easily be distinguished by its lacinated or deeply cut leaflets as opposed to the whole serrated leaflets of Himalayan Blackberries.
• Rubus pascuus Chesapeake Blackberry is a native shrub of Kentucky, Virginia, Maryland, Delaware, North Carolina, and South Carolina. It has tighter and shorter inflorescences, is more densely glandular, and its terminal leaflets are erect rather than spreading in Rubus bifrons.
• Rubus pensilvanicus Pennsylvania Blackberry is native throughout eastern Canada and the USA. It usually is a bit smaller than the Himalayan Blackberries, and its very red stems are densely covered in red prickles. It also only ever has white petals on its flowers, and its abaxial leaf sides are hairy but never whitish or grayish-green.
• Rubus ulmifolius Elmleaf or Thornless Blackberry is another European species that has been introduced that superficially looks very similar, but it is only found in California, Oregon, and Nevada. If prickles are not present, they can easily be distinguished by this. However, sometimes it does have prickles, in which case you can differentiate it by its strongly pruinose stems that Himalayan blackberries lack.
• Rubus vestitus European Blackberry is found on the Pacific coast of the US and Canada. It is an armed shrub that is generally smaller than 2 m. It also has narrower inflorescences, nearly round terminal leaflets, and stipitate-glandular hairs lacking in Himalayan Blackberries.

Native Distribution of Rubus bifrons or Rubus armeniacus

There is much debate as to the taxonomic description of this species. Some sources call them all Rubus armeniacus and say these are from Europe. Other sources say that the species are only native to Armenia and Iran. Other sources call this plant Rubus bifrons and say it is widespread in Europe and includes Rubus armeniacus. In either case, whether the same or separate species, they are virtually identical, and both are native to Europe, one more widespread and the other less so.

Habitat Types Where Himalayan Blackberry is Found

Himalayan Blackberry grows in both wetland and upland environments. While it grows abundantly in riparian areas, it does not invade the permanently wet soils of the wetlands though it will tolerate temporary flooding of up to 40 days. It will even tolerate occasional flooding of brackish water. It grows best at low elevations but can grow up to 1200 and occasionally 1800 m above sea level.

Himalayan Blackberries prefer moist, nutrient-rich loam but do not require it. They can also be found growing in soils with a wide range of textures, fertilities, and pH from acid to alkaline. It prefers sunny sites but easily grows in part shade as well.

Human Uses of Himalayan Blackberry

Himalayan Blackberries are widely used as food. They are eaten fresh as berries or frozen or canned for winter use in pies, cakes, jams, jellies, and wines.

Blackberry roots and berries are often used to treat diarrhea, fluid retention, gout, diabetes, gout, and inflammation. It is sometimes used to prevent heart disease and cancer. It makes a good rinse for irritation of the mouth and throat.

They are planted along fences and trellises to create impenetrable barriers to keep people and animals out of an area.

Distribution of Rubus armeniacus, Rubus bifrons in North America

The Himalayan Blackberry species were first brought to North America as a food crop in 1885 for its abundant berries.

In Canada, Rubus bifrons has been recorded in British Columbia, Ontario, Quebec, and Nova Scotia. Canadensys does not distinguish between the two species and uses only Rubus bifrons stating that Rubus armeniacus is a synonym.

In the USA, the Himalayan Blackberry is reported as two separate species. USDA states that Rubus bifrons is only found in the eastern part of the country in Oklahoma, Texas, Missouri, Arkansas, Louisiana, Kentucky, Tennessee, Mississippi, Alabama, Georgia, South Carolina, North Carolina, Maryland, Virginia, Washington DC, Pennsylvania, New Jersey, New York, Connecticut, Rhode Island, and Massachusetts.

USDA states that Rubus armeniacus is more widespread in the west and scattered in the east. It is found in Washington, Oregon, California, Idaho, Montana, Nevada, Arizona, Colorado, New Mexico, Missouri, Arkansas, Illinois, Kentucky, Tennessee, Alabama, Ohio, Virginia, Delaware, Pennsylvania, New Jersey, and Massachusetts.

In Mexico, Rubus bifrons so far has been reported in Veracruz and Oaxaca. Rubus armeniacus has been reported in Baja California (norte), Puebla, Mexico State, Jalisco, and Chiapas.

Himalayan Blackberries have now been introduced on every continent except Antarctica.

How Himalayan Blackberry Spreads

It is primarily spread long-distance by deliberate human introductions planting it in their gardens. It quickly escapes from cultivation due to its aggressive growth.

Short-distance dispersal occurs through birds and animals which eat the abundant fruit and then spread the seeds around in their feces. Humans also spread the fruits by picking them, eating them and dropping them, etc. They also spread vegetatively through their root crown and rhizomes and by rooting at nodes on their stems that come into contact with soil.

Habitats at Risk of Invasion in North America

Himalayan Blackberries are considered particularly invasive on the west coast of North America in low-elevation riparian, deciduous, and conifer forests as well as grasslands. It is heavily invasive in riparian communities in that area. It invades the same types of areas in the east but may be less invasive there due to the other aggressive native and non-native vines that are abundant in that region.

Himalayan Blackberry will not invade permanently wet soils or high-elevation montane forests. It will grow in middle-elevation montane forests but not very well. It will also not grow in most arid areas due to lack of moisture unless it is in a riparian zone with perennial access to water.

Impacts of Invasion

In North America, the Himalayan Blackberry is considered the most invasive nonnative shrub on the western coast from British Columbia, Canada, south to California, USA. They are aggressively spreading shrubs that quickly scramble over and smother all other vegetation in their path, excluding tall trees, as they cannot climb as high as English Ivy, for example. However, it can quickly smother and kill all young trees in its path. It is highly competitive for light, space, and nutrients. Due to its aggressive growth, it quickly out-competes native vegetation.

It replaces the more bio-diverse native shrubs and herbs that would have otherwise grown there, creating complete monocultures if the conditions are suitable for them. This out-competes our native vegetation, reducing biodiversity and even causing local extirpation of species. It also forms dense thickets in riparian areas that impede movement and access to water by humans and animals alike.

Himalayan Blackberry is also an agricultural and range land weed that replaces crop and forage lands quickly if left unchecked. Himalayan Blackberry and Scotch Broom are the two most abundant agricultural and range land weeds, causing an estimated $85 million in losses annually.

The abundant dead canes they produce are also a significant fire risk when they die and become brittle as they burn easily.

Potential Benefits of Invasion

Himalayan blackberries are widely eaten by bears, foxes, coyotes, squirrels, raccoons, deer, birds, slugs, and countless other native species. Most native ungulates prefer to browse the leaves, but some (white-tailed deer, for example) have been observed browsing the fruits. Leafcutter ants and leafhopper butterfly larvae also feed on the leaves. It is the preferred larval host plant for leafhopper butterflies. Pollen and nectar are also used by native bees, honey bees, and hummingbirds.

In addition, a multitude of small animals and birds use the blackberry thickets for cover, nesting, roosting, etc. The impenetrable thickets of thorns provide them protection from predators.

All of these animals, however, would have fed on the native Salmonberries, Thimbleberries, or other native species displaced by the invasive Rubus bifrons, however. The dependence of wildlife on the berries creates an additional challenge to the control or removal of the Himalayan Blackberries. Check out the control methods below for more information.

Methods to Remove Himalayan Blackberry

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in many local garden stores. Do not buy or transport any Himalayan Blackberry. Do not plant it in your yard.

If you see them being sold online or in your local garden stores, please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Instead, buy suitable native Rubus species. There are multiple native blackberries and raspberries that are even more delicious and are just as easy to grow as invasive blackberries. Native Rubus species provide additional biodiversity and wildlife values as well.

In fact, due to wildlife values, this makes the removal of Himalayan Blackberry more challenging. Because native wildlife now depends on this non-native species, when you remove a large patch, it should be done in stages. Clear one-third or one-quarter of the patch (depending on the size).

Once cleared and few sprouts keep coming back, then it should be replanted with a variety of native salmonberries, thimbleberries, or other native shrubs that would have originally fed the wildlife in your area.

Research what species are native to your area to find suitable replacements. Then tackle the next fraction of the patch until you have replaced the invasive blackberry with a more biodiverse native selection.

Ongoing monitoring will, of course, be needed to ensure it does not return and overtake your planted native species. See the Integrated Management section below.

In public and natural areas minimizing disturbance and replanting disturbed areas as quickly as possible are the best prevention strategies for reducing the chance of invasion by Himalayan Blackberry or any other invasive species.

Physical Control of Himalayan Blackberry

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming, but it usually causes the least amount of environmental damage.

The best time to remove Himalayan Blackberry is early in the spring, before or later during flowering but before the fruit has set to avoid further spreading the seeds.

Physical methods to remove Himalayan blackberries generally involve cutting, hoeing, digging, and burning. Often a combination of methods is used. Always wear gloves when handling the canes, as the prickles are very tough and sharp and will easily pierce the skin.

Young plants can be pulled by hand.

Mature plants require the cutting of the canes with cutters or loppers and then removing the rootstock.

Burning is also an effective method to remove the above-ground growth, but as with cutting, the rootstock will need to be dealt with as well. A single cutting back of the above-ground growth, however, if not followed with additional treatments, will actually increase the density of the thicket when it regrows. It must be followed with additional treatments and ongoing monitoring.

If the rootstock is not removed, it will grow back quite vigorously. One method is to repeatedly cut it back or burn it multiple times over multiple years until the roots are starved by the lack of above-ground growth.

Rootstocks can also be dug out with a shovel or a weed puller, but they are very tough, and this is only suitable in small patches.

If you have only a small patch to deal with and root digging is too labor-intensive, you could also cover the entire area with a black tarp for at least two growing seasons to solarize it. This is much less labor-intensive than returning repeatedly, but again this only works on small patches.

Note that it is virtually impossible to remove all of the rootstocks due to the size and depth that the roots spread to. Removing as much as you can is critical to success. However, it must be followed by ongoing monitoring over several years to ensure all new sprouts are destroyed.

Disposal of the Shrubs Once Removed

If you have live stems or plants that have seeds on them, they must either be burned in a hot enough fire or solarized. The fire must be hot enough to destroy the seeds. If you are not allowed fires in your area, then you will have to solarize them.

To solarize, put the shrubs under a thick black tarp or into thick black garbage bags and leave them in the full sun for a good 8 – 10 weeks at least to be sure that all seeds are no longer viable. Some sources suggest less time for polarization, but in my experience, differential heat throughout the bag/tarped area as well as variations in sunlight intensity, can result in the survival of at least some seeds if done for a shorter duration. Take them to your local dump once done and inform them that they are invasive species so that they are disposed of appropriately.

Chemical Control of Himalayan Blackberry

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used, species riches significantly increase.

Furthermore, there are no chemical control methods that effectively target only Rubus bifrons or Rubus armeniacus. And due to the nature of Himalayan Blackberry, multiple applications are always needed over multiple years, further degrading the local environment.

Chemical control is not recommended.

Biological Control of Himalayan Blackberry

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day, there are still rats in Hawaii, but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. This is because the control agent would need to effectively destroy over 99% of plants, roots, and seeds to actually control the Himalayan Blackberry on their own. Results this high have never been achieved in the field.

However, using biological control in conjunction with physical control and ongoing monitoring can be very effective at times. However, in North America, so far, there are no biological control methods in terms of insects, fungi, or other pathogens that have been approved. This is because they all put our abundant list of native Rubus species at risk as well.

Using Goats to Remove Himalayan Blackberries

The only recommended biological control method at this time is grazing by domestic goats. Domestic goats are aggressive and non-selective browsers. They will literally eat anything. To use goats, the patch should be penned in, and then the goats left there to graze for 3 years. They will eventually destroy all the above-ground growth and then eat the new sprouts as they come up.

This can be sped up by an entire year by first removing the above-ground growth and then allowing the goats to eat the sprouts as they come up.

This can then be followed with rootstock removal for any stubborn plants that may remain, or simply leave the goats in the area for another year until you see no more sprouts.

At this time, they can be moved to another patch, and that patch can be replanted with native vegetation and a monitoring program implemented to be sure they do not sprout again.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts or seedlings otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Crucial

In all cases of Himalayan Blackberry removal, the site will need to be replanted once the majority of the infestation has been dealt with. This is because the bare soil will allow the seed bank of Himalayan Blackberry in the soil to germinate and re-invade the patch they were removed from. Or it will allow the invasion by other nonnative species, which all favor disturbed sites.

A replanting program should already be planned and ready to implement in the 2nd to 4th year upon removal of the Himalayan Blackberry, depending on the speed and success of the removal method.

A variety of native species that are ideally seeds from the local environment to ensure local ecotypes should be planted. If local seeds cannot be gathered or started in time, then purchasing native plants from a reputable nursery specializing in local native species is a good alternative. This must then be followed by ongoing monitoring.

Ongoing Monitoring is Essential

In all cases of invasive Himalayan Blackberry removal, ongoing monitoring is absolutely essential.

Multiple visits should be made in the first two years to ensure that any surviving individuals, their sprouts, and their seedlings are removed so that the population is not able to recover.

Then yearly monitoring after that for at least five years to ensure that none return. This is required whether the area is replanted or not. Himalayan Blackberry is aggressive and prolific and will out-compete planted vegetation if yearly monitoring is not put in place to remove young plants before they have a chance to become established.

References and Resources

CABI on Rubus armeniacus https://www.cabi.org/isc/datasheet/116780

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Eflora Plants of North America http://www.efloras.org/browse.aspx?flora_id=1

Fire Effects Information System on Himalayan Blackberry https://www.fs.fed.us/database/feis/plants/shrub/rubspp/all.html

iNaturalist Plant Search https://www.inaturalist.org/home

Rubus armeniacus fruits picture from Wikipedia By Daderot – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=21796138

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia Himalayan Blackberry https://en.wikipedia.org/wiki/Rubus_armeniacus

Willis, Lyrae (Unpublished).  Plant Families of North America.

Currently Seeking Funding To Continue This Non-Profit, Ad-Free Work

If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

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Introduction

I never saw saltcedar, Tamarisk, or any of the other common names that Tamarix species go by until I traveled south. So far, they have not ventured far into Canada and certainly haven’t been venturing there long. However, in the south, they thrive in hot, dry areas, particularly saline soils where few plants will grow, allowing them to take over. There are 50 – 60 species of Tamarix in the world, and not one is native to North America. In North America, we have Tamarix aralensis, Tamarix aphylla, Tamarix africana, Tamarix canariensis, Tamarix chinensis, Tamarix gallica, Tamarix parviflora, Tamarix ramosissima, and Tamarix tetragyna.

All Tamarix species should be considered invasive in North America. They have covered over a hundred million acres in the western US alone, and they frequently hybridize, making identification to the species level challenging. For the purpose of identifying them as an invasive species and for Tamarisk or Saltcedar removal or control, identification to the genus level is more than sufficient.

Description of Tamarix spp

Leaves & Stems of Tamarix spp

Tamarix spp are part of the Tamaricaceae family in the Caryophyllales Order of flowering dicot plants. They are either shrubs or small trees from 1- 10 m tall and are often multi-stemmed. They are a long-lived species living 100 years or more. Most species are deciduous. They are almost all very deep-rooted, with a tap root up to 30 m deep that also produces lateral roots up to 50 m long that can produce adventitious buds, especially when covered by shifting sand. Their deep taproots are meant to reach down into the groundwater, allowing them to survive in hot drought-prone climates as long as they are close to the water table.

Young branches are glabrous and are a variety of colors depending on the species. They may be reddish-brown, brown, blackish-brown, dark purple, grey, or black. Older branches have heavy bark that is frequently shredded and may be grey or brownish and reaches 10 – 15 (-30) cm in diameter.

The usually sessile (without leaf stalks) leaves of Saltcedars are much reduced and appear small and bract-like, somewhat resembling that of coniferous tree needles. Leaves are varying shades of dull, light, or dark green and turn golden-orange in the fall.

Flowers & Fruits of Tamarix spp

Tamarisk or Saltcedars produce a racemose inflorescence that is often paniculately branched. They are borne on the ends of current-year branches or in some species’ previous-year branches. In most species in North America, the flowers appear after the leaves have grown back. The flowers are very small, usually pink, and most have small petals only 1 – 2 mm long.

Fruits are multi-sided capsules with up to thousands of tiny seeds. The capsules usually have a tuft of hair that aids in wind dispersal. They can also be dispersed in water. Seed viability is very short, however, from around 20 to a maximum of 120 days, depending on the weather. However, Saltcedars can flower and produce seeds for extended periods of time, allowing them to reproduce prolifically even with their short seed life.

Similar Species Tamarix spp are Frequently Confused With

Other Genera

There are a few other genera that Tamarix species are occasionally confused with that also grow in similar habitats and ranges and have much-reduced leaves. However, all of those can easily be differentiated by their flowers.

Parkinsonia species are members of the Fabaceae family and have larger yellow, somewhat pea-like flowers and produce small legume fruits.

Prosopis glandulosa has sprays of yellow rather than pink flowers, and it is also a Fabaceae that produces long legume fruits.

Lepidospartum squamatum has a limited range native to the southwestern USA and northwestern Mexico and has small yellow Asteraceae-type disk flowers and typically Asteraceae cypsela fruits. Baccharis neglecta has a similar North American range, but it is a perennial plant, not a shrub, and produces individual whitish flowers more typical of the Asteraceae family it belongs to.

Polygonella robusta is an uncommon plant of the Polygonaceae family that is endemic to Florida and produces similar sprays of small pink flowers, but its leaves, while still small, are long and linear compared to all Tamarix species.

Juniperus virginiana smells strongly of Juniper and has characteristic powdery blue “berries” and no pink flowers.

Casuarina equisetifolia is widespread in Mexico and more limited in the southern USA. It has needle-like leaves, but they are longer, divided into septa, and grouped in fascicles, and it produces a globose cone-like fruit.

Tamarix Genera

Tamarix species can be challenging to identify at the species level. For the most part, genus-level identification in North America is sufficient for treating the plants as invasive species, as none are native to North America. However, with a bit of patience and a small amount of skill, they can be identified to the species level as follows:

• Tamarix aralensis Russian Tamarisk has a limited range in North America. It is similar to T. chinensis and T. ramosissima but has a much more limited range. It can be differentiated by its petals that fall off at the time of seed maturation.
• Tamarix aphylla Athel Tamarisk is more widespread in Mexico than in the southern USA. It is the only evergreen species, and its leaves are sheathing on the stems rather than sessile or amplexicaul like all other Tamarix species in our area. It also produces white instead of pink flowers.
• Tamarix africana African Tamarisk has a very limited range in the southern USA. It has the largest flowers (though still small) with petals that are 2 – 3 mm long and racemes that are 5 – 9 mm wide.
• Tamarix canariensis Canary Island Tamarisk has a limited range in the southern USA. It is similar to T. chinensis and T. ramosissima, but its sepal margins are denticulate, and it has obovate petals that are 1.2 – 1.5 mm long.
• Tamarix chinensis Five-Stamen Tamarisk is widespread in Mexico and the USA. It also is the most similar to T. ramosissima, and the two often hybridize, leading some to believe they should be considered the same species. They have five petals and five stamens. They can only be differentiated by this one’s sepal margins that are entire and that some or all of the stamens’ filaments originate from below the nectar disc. Filament inspection requires patience, a hand lens, and some skill. Furthermore, its petals persist at seed maturation, unlike T. aralensis.
• Tamarix gallica French Tamarisk has a limited west and south range in the USA and northern Mexico. It is also similar to T. chinensis and T. ramosissima, but its sepal margins are entire or almost entire, and it has elliptic to ovate petals that are 1.5 – 2 mm long.
• Tamarix parviflora is very widespread in the USA and is limited to the northern parts of Mexico. Its flowers are produced before it gets its leaves, and it usually only has four petals, unlike all other species in North America, whose flowers are five-petaled and arrive after the appearance of leaves.
• Tamarix ramosissima is fairly widespread in both USA and Mexico. It is the most similar to T. chinensis, and the two often hybridize, leading some to believe they should be considered the same species. They have five petals and five stamens. However, this species’ sepal margins are denticulate rather than entire, and all the stamens’ filaments originate from the edge of the nectar disc rather than below it in T. chinensis. Filament inspection requires patience, a hand lens, and some skill. Furthermore, its petals persist at seed maturation, unlike T. aralensis.
• Tamarix tetragyna Four-Stamen Tamarisk is found only in Georgia. It can be differentiated by its flowers that have only four stamens as opposed to the five that all other species in North America have.

Native Distribution of Tamarix spp

There are approximately 100 species of Tamarix, and all are native to Eurasia and Africa. None are native to the Americas.

Habitat Types Where Saltcedar is Found

Saltcedar, with its long taproots, is usually found only near groundwater and is, therefore, particularly common in riparian habitats, including floodplains, permanent or ephemeral stream banks, and around lakes and water reservoirs. They can also grow in upland habitats out of contact with groundwater, but they are less common there and grow less vigorously there. They can grow from 0 – 2000 m above sea level.

There is a common misconception that they are saline-loving plants, but in fact, they tolerate a wide range of soils, including those fairly high in salinity. This gives them a significant competitive advantage in saline areas where most other plants cannot survive. In those conditions, they typically rapidly produce monocultures displacing the few native halophytes found there.

Human Uses of Tamarisk

Tamarisk is used as an ornamental, especially in marginal habitats or saline soils. It was also widely planted as a stream and dune stabilizer in some areas. It is being used in China to fight the expansion of deserts.

Tamarisk wood is sometimes used for firewood, carpentry, and the making of bows and other tools.

In Africa and Asia, Tamarix species are used medicinally to treat wounds and infections as well as liver and spleen disorders. It is seldom currently used in North America medicinally.

Distribution of Tamarix spp in North America

The species was first brought to North America in the 1800s as an ornamental species. There are no species of Tamarix native to North America.

In Canada, Tamarix ramosissima has been recorded but ephemeral in British Columbia. It has been seen but not lasted long enough to confirm in Ontario, Manitoba, Quebec, and Nova Scotia.

In the USA, nine Tamarix spp are found. They are distributed as follows:

• Tamarix aralensis Russian Tamarisk is found in California and North Carolina.
• Tamarix aphylla Athel Tamarisk is found in California, Nevada, Utah, Arizona, and Texas.
• Tamarix africana African Tamarisk is found in California, Arizona, Texas, Louisiana, and South Carolina.
• Tamarix canariensis Canary Island Tamarisk is found in Arizona, Louisiana, Georgia, South Carolina, and North Carolina.
• Tamarix chinensis Five-Stamen Tamarisk is more widespread and is found in Washington, California, Nevada, Utah, Arizona, Montana, Wyoming, Colorado, New Mexico, Oklahoma, Texas, Arkansas, Ohio, and North Carolina.
• Tamarix gallica French Tamarisk is found in Washington, California, New Mexico, Texas, Louisiana, Georgia, South Carolina, and North Carolina.
• Tamarix parviflora is the most widespread species in the USA, being found in Washington, Oregon, California, Idaho, Nevada, Utah, Arizona, Montana, Colorado, New Mexico, Kansas, Oklahoma, Texas, Missouri, Louisiana, Illinois, Mississippi, Kentucky, Tennessee, Florida, North Carolina, Virginia, Michigan, Pennsylvania, Delaware, New Jersey, Connecticut, and Massachusetts.
• Tamarix ramosissima is also fairly widespread, being found in California, Nevada, Utah, Arizona, Colorado, New Mexico, North Dakota, South Dakota, Nebraska, Kansas, Oklahoma, Texas, Arkansas, Louisiana, Mississippi, Georgia, South Carolina, North Carolina and Virginia.
• Tamarix tetragyna Four-Stamen Tamarisk is found in Georgia.

In Mexico, there are five species of Tamarix found as follows:

• Tamarix aphylla is the most widespread found in Baja California, Baja California Sur, Sonora, Sinaloa, Chihuahua, Durango, Jalisco, Michoacan, Coahuila, Nuevo León, Tamaulipas, San Luis Potosi, Guanajuato, and Mexico City.
• Tamarix chinensis is found in northern Mexico in Baja California, Baja California Sur, Sonora, Sinaloa, Chihuahua, Coahuila and Nuevo León. It is possibly found in Mexico City, Mexico State, and Tamaulipas, but this is unconfirmed.
• Tamarix gallica is much less widespread and also only found in the northern states of Baja California, Coahuila, and Nuevo León. There is also an unconfirmed location in Chihuahua.
• Tamarix parviflora is the least widespread in Mexico and is only found on the northern border of Baja California. It is not far from the border in New Mexico, so it may likely spread to Chihuahua from there soon.
• Tamarix ramosissima is the second most widespread in Mexico, with populations found in Baja California, Baja California Sur, Sonora, Sinaloa, Chihuahua, Coahuila, Nuevo León, Guanajuato, Queretaro, Mexico State, Mexico City, and Guerrero.

How Tamarisk Spreads

It is primarily spread long-distance by deliberate human introductions into gardens or as a landscape plant.

Short-distance dispersal occurs through the abundant tiny seeds spread by wind, water, humans or animals. Seeds have a very short viability period; however, if they are not germinated during the summer that they are dispersed, almost none germinate the following year. However, Saltcedar produces seeds over a very long period enabling its rapid spread. Mature tamarisk plants also spread vegetatively by adventitious roots producing colonies of clones. 

Habitats at Risk of Invasion in North America

All riparian areas are at risk of invasion, particularly in the semi-arid and arid southern areas of our continent. More northern areas are less at risk, but we have seen invasion into North and South Dakota, Washington State, and New York, as well as the so-far ephemeral invasion into southern Canada. Tamarisk or Saltcedar has expanded its riparian dominance significantly since the 1800s and now covers huge geographical areas. Its rapid spread does not seem to be slowing, and its range continually grows.

In California desert areas, Saltcedars have now even become established in remote mountain springs, streams, and washes. These areas show no sign of human disturbance and are long distances away from any source of infestation. This means that wild riparian areas far from humans are not safe from Saltcedar invasion.

Impacts of Invasion

Tamarisk thrives in riparian habitats where it quickly displaces native vegetation due to rapid growth, tolerance of a wide range of environmental conditions, and long periods of abundant seed production. Tamarisk also accumulates salt in its leaf glands and excretes it onto the leaf surface. These salts accumulate in the soil when the plants drop their salty leaves. As surface soils become more saline over time, they further exclude native vegetation, producing monocultures. Native herbivores’ preference for native plants gives Tamarix species yet another competitive advantage.

Significant loss of biodiversity occurs in the stands of Saltcedar. Plant diversity is dramatically reduced, resulting in a loss of riparian bird, insect, invertebrate, aquatic and animal diversity.

Potential Benefits of Invasion

Saltcedar provides a habitat for a number of bird species. However, many of these are introduced from the Old World, where Tamarix evolved; though some of our native birds do use it, they still prefer native plants.

Methods to Remove Tamarix spp

As always, prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in many local garden stores. Do not buy or transport any Saltcedar. Do not plant it in your yard. There are so many Native Plant Species that would make suitable garden specimens that are noninvasive and provide additional wildlife and biodiversity values.

If you see Tamarisks being sold online or in your local garden stores, please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species.

Physical Control of Saltcedar Tamarisk

Once already established, however, physical control is a labor-intensive but effective means to remove Saltcedar. Physical control is time-consuming and costly, but it usually causes the least amount of environmental damage and leaves the area the most biodiverse afterward.

The best time to remove Saltcedar or Tamarisk is before it has gone to seed. Because it flowers and seeds for an extended period of time, this must be done in the late fall, winter, or early spring. If the plants are in seed upon removal, they can be burned or solarized as the seeds are sensitive and should die easily.

Physical methods to remove Saltcedar generally involve pulling young plants by hand, if very small, or with a weed puller or digging them out with a hoe or shovel.

Mature specimens are generally cut down, but the rootstock must be either dug out with a machine or chemically treated for a period of time to prevent re-sprouting. Isolated larger specimens can be solarized to prevent re-sprouting, but the tarp must remain for 2 growing seasons. Another method is to simply keep returning to the area a few times each growing season for several years in a row to keep cutting any new sprouts. This will eventually starve out the roots and kill the plant.

Along controlled rivers scheduling 5 – 10 year floods which wash out the Saltcedars removing them and leaching salts from the soil. This allows the native vegetation adapted to periodic flooding to grow back and displace the Saltcedar. The seedlings can be easily killed by keeping an area flooded for one month. The flooding method can also be used in low-lying areas that are isolated. Cut down all mature plants to the ground and keep the area flooded for an extended period of time. It will kill or prevent any new seedlings or sprouting and will eventually start rotting out and starving the roots. After the water is allowed to drain, the short seed viability period makes it unlikely that many new seedlings will emerge. Ongoing monitoring and hand-pulling of the few that do grow can be effective at preventing the population from becoming re-established.

Methods Not Recommend

Neither mowing nor burning will effectively control mature Saltcedar. Plants will quickly regrow from their basal stem buds and be supported by their extensive and deep root systems. Seedlings can be controlled by physical removal as mowing is generally ineffective on them as well due to their roots.

Disposal of the Shrubs Once Removed

If you have plants that have seeds on them, they must either be burned or solarized. Though mature plants are fire-adapted and will regrow, their sensitive seeds are vulnerable to fire, so the burning of shrubs is desirable in areas where burning is allowed.

If burning is not allowed, you should solarize the shrubs under a thick black tarp or in thick black garbage bags and leave them in the full sun for 6-8 weeks to be sure that all seeds are no longer viable. Since seed viability is poor, solarizing is also an effective method to destroy seeds.

Chemical Control of Saltcedar Tamarisk

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used, species riches significantly increase.

Saltcedar is somewhat resistant to chemical control methods, with up to 30% of plants resprouting up to three years after treatment. This means multiple applications will be needed. Furthermore, there are no chemical control methods that effectively target only the control of Saltcedar. Finally, due to their preference for riparian habitats, the use of herbicides is often not allowed due to proximity to water.

However, professionals and government officials may determine where and when chemical applications can be safely carried out to deal with vast areas of infestation. Chemical control by individuals is generally not recommended.

Biological Control of Saltcedar

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day, there are still rats in Hawaii, but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. This is because the control agent would need to effectively destroy over 99% of seeds to actually control the Saltcedar on their own. Results this high have never been achieved in the field. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective.

So far, in North America, only one biological control agent has been released to control Tamarisk. Diorhabda elongata deserticola is a leaf beetle from western China and eastern Kazakhstan that was released in the USA in 1986. The beetles established well in the more northern areas and, after the third season, were seen to defoliate over 95% of the plants in the area. The populations have been spreading and are successfully helping to control Saltcedar in larger and larger areas. The introduced leaf beetle has also shown to be highly selective for Tamarisk and so far has not been observed eating any native vegetation. This makes it highly suitable for the biological control of Saltcedar.

The leaf beetles, however, did not establish in the southern areas due to increased predation and climatic differences. Additional subspecies of D. elongata have now been obtained from lower latitudes in Europe and Asia to be tested in more southern locations. It is too early for conclusive results.

Goats and cattle will graze on Saltcedar, but cattle, in particular, will selectively graze on native vegetation where it is present. Goats are less selective and will generally eat anything. Grazing would need to be done on a continual basis for at least 4 – 5 years so that they eat all sprouts that try to grow back up from the rootstock. Eventually, this would starve out the rootstock, and the plants would die.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form, this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. Integrated management is required because the area needs to be monitored for returning sprouts or seedlings; otherwise, all the hard work done in removal could be wasted if the invasive species is allowed to regrow.

Replanting With Native Species is Crucial

In all cases of removal, the site should be replanted once the area has been cleared of Saltcedar and the soil has been leached of excess salts. Leaching the soil involves manually flooding the area with fresh water if there is insufficient rain in the region. Otherwise, waiting until after the rainy season may be sufficient as long as there are no more Saltcedars there.

Bare soil invites more infestations, and high salinity prevents biodiversity from improving. A replanting program should be planned and ready to implement the following year after the removal of Saltcedar. Then ongoing monitoring will be necessary to ensure no sprouts survive and no new seeds are accidentally brought into the area.

Ongoing Monitoring is Essential

In all cases of invasive Saltcedar removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving sprouting individuals are removed so that the population is not able to recover. Due to the short viability of seeds, new seedlings will stop emerging after a single season, provided no new seeds are accidentally transported to the site. Monitoring is required whether the area is replanted or not. Saltcedar has very deep and widespread roots that will easily regrow and start a new infestation if left unchecked. Yearly monitoring will prevent this and will allow the site to recover so that native plants will return.

References and Resources

CABI on Tamarix ramosissima https://www.cabi.org/isc/datasheet/52503

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Eflora Plants of North America http://www.efloras.org/browse.aspx?flora_id=1

Fire Effects Information System on Tamarix spp https://www.fs.fed.us/database/feis/plants/tree/tamspp/all.html

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia Tamarix https://en.wikipedia.org/wiki/Tamarix

Willis, Lyrae (Unpublished).  Plant Families of North America.

Currently Seeking Funding To Continue This Non-Profit, Ad-Free Work

If you are able to donate so that I can continue this non-profit work of supplying people with scientific information on the plant families, native plants, and invasive species found throughout North America, please donate using the GoFundMe link below. Thank you!

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